Whether you and your horse barrel race, practice dressage, or simply enjoy time out on the trails, your horse uses their eyes to maneuver obstacles to keep themselves and you safe. Tumors that begin inside a horse’s eye are rare, but growths commonly target the sensitive tissues surrounding their eyes. These tumors can progress to invade the horse’s cornea and internal eye structures, threatening their sight. Our Veterinary Vision Center team provides information on these tumors to help you recognize a problem before your horse’s vision is affected.

How does squamous cell carcinoma affect my horse’s eye?

Squamous cell carcinoma (SCC) frequently affects the tissues around the horse’s eye. If left untreated, your horse could lose their sight, or may need the eye removed.

  • SCC occurs most frequently in horses whose eyelids lack, or have only light, pigmentation.
  • SCC usually affects horses 8 to 10 years of age, but the lesions can develop at any age.
  • Horses exposed to excessive ultraviolet light are at higher risk.
  • In North America, horses living in western and southern states are more at risk.
  • Drafts, Appaloosas, and Halflingers are at higher risk.
  • Signs include squinting, and runny and red eyes, but horses may also show no discomfort.
  • Initial lesions appear as red, bumpy tissue on the eyelid, third eyelid (i.e., the triangular-shaped tissue at the inner corner of the horse’s eye), or limbus (i.e., the area where the cornea meets the white tissue).
  • Lesions can progress to large, draining growths.
  • Diagnosis is by biopsy.
  • Invasion onto the corneal surface is common.
  • Early and complete excision combined with radiation or chemotherapy is the most successful means to prevent recurrence.

Unfortunately, recurrence is common after treatment. A UV-protectant fly mask can help prevent squamous cell carcinoma if used from a young age. If your horse lacks, or has limited pigment, around their eye, the mask should be worn year-round. Initial lesions are easy to miss because they can manifest as slight reddening to the tissue surrounding the eye, which you may dismiss as fly irritation. If the growth invades the cornea, the eye takes on a cloudy appearance and may tear slightly. Examine your horse’s eyes carefully every day to recognize any abnormalities that need evaluation by our veterinary team.

How do sarcoids affect my horse’s eye?

Sarcoids are the most common tumors that affect a horse’s skin, often appearing as a lump under the eyelid. Horses will typically have several of these tumors, which come in various forms and can change forms if aggravated, over their body. Sarcoids are benign fibrous tumors, but are locally aggressive and can be problematic if not treated promptly.

  • Sarcoids are thought to be associated with the bovine papillomavirus.
  • Initial lesions appear as a lump under the eyelid, or as wart-like lesions, depending on the form.
  • Lesions can progress rapidly, invading the skin and transforming to a red, fleshy mass.
  • Diagnosis is by biopsy.
  • Treatment should begin soon after the biopsy, because trauma can cause the tumor to become more aggressive.
  • Treatment options include laser surgical removal, chemotherapy, radiation, cryotherapy, and immunotherapy.
  • A sarcoid vaccine that has proven effective is available. After removal, small sarcoid pieces are implanted in the horse’s neck.

Like squamous cell carcinoma, sarcoids are notorious for their recurrence, so addressing them early when they are small is important. 

How do melanomas affect my horse’s eye?

Melanomas affect more than 80% of grey horses, frequently occurring around their eyes. Melanomas on non-grey horses are not common and tend to be more malignant.

  • Melanomas are genetically linked to the grey gene.
  • Initial lesions appear as small, solid, spherical lumps under the skin.
  • Growth can be slow for years and then suddenly accelerate.
  • Diagnosis is by biopsy.
  • Grey horses affected by melanoma typically have several over their body, and sometimes internally.
  • Treatment options include surgical removal and cryotherapy.

A melanoma vaccine is available that was initially approved for dogs, but has been used successfully in some horses and resulted in stabilizing the growth rate. Unfortunately, the treatment is not curative, and attempting the therapy is a gamble, because the cost is substantial and results are not guaranteed.

How do lymphomas affect my horse’s eye?

While the eye is typically a secondary lymphoma location after metastasis, primary lesions have been observed.

  • Lymphoma is the most common cancer-causing death in horses.
  • Lesions can affect one or both eyes.
  • Lesions appear as red, thickened tissue around the eye margin.
  • Diagnosis is by biopsy.
  • If the eye is the primary site, removing the eye and surrounding tissue can be curative.
  • If the eye is the secondary site, long-term survival is unlikely. The prognosis is especially poor if the horse has fluctuating fevers and changes in bloodwork.

Your horse’s eyes are a precious feature that should be fiercely protected. Examine your horse’s eyes frequently so you are familiar with their normal structures and will immediately recognize a troublesome growth. Do not hesitate to contact our team at Veterinary Vision Center if you are concerned about a lesion, or any other abnormality, on your horse’s eye.